PNNL

Abstract

Direct electrochemical upgrading of CO2 in capture media is an attractive approach to carbon capture that can bypass the energy requirement for the thermal release of pure CO2. Here we investigate the electrocatalytic activity of iron(tetraphenylporphyrin) in the presence of organic solvents that convert into ionic liquids upon exposure to CO2. Four different solvent systems were tested, all of which capture CO2 in the form of an alkyl carbonate (or carbamate) anion and an acidic ammonium cation. The electrocatalytic selectivity exhibited a strong dependence on the acidity of the capture medium, with the most basic solvent affording a high selectivity for production of CO instead of H2. Experimental and computational studies support a canonical mechanism in which the catalyst reacts with free CO2 in solution, as opposed to a reaction with the alkyl carbonate that is present in high concentration. Kinetic analysis indicates that the rate-limiting step is changed from C–O protonolysis in traditional solvents to the binding of CO2 in the capture media. Quantitative 13C–13C EXSY revealed that the dissociation of the alkyl carbonate into free, solvated CO2 is very rapid (~15 s–1) compared to the interconversion of HCO3–/CO2 in aqueous solution. These results underscore the need to understand the mechanism and kinetics for both the release of captured CO2 and its electrocatalytic conversion.